Fluorocarbon based fluids have found widespread use in industry for refrigeration, air conditioning and heat pump applications. Vapor compressions cycles are one form of refrigeration. In its simplest form, the vapor compression cycle involves changing the refrigerant from the liquid to the vapor phase through heat absorption at a low pressure, and then from the vapor to the liquid phase through heat removal at an elevated pressure. First, the refrigerant is vaporized in the evaporator which is in contact with the body to be cooled. The pressure in the evaporator is such that the boiling point of the refrigerant is below the temperature of the body to be cooled. Thus, heat flows from the body to the refrigerant and causes the refrigerant to vaporize. The formed vapor is then removed by means of a compressor in order to maintain the low pressure in the evaporator. The temperature and pressure of the vapor are then raised through the addition of mechanical energy by the compressor. The high pressure vapor then passes to the condenser whereupon heat exchange with a cooler medium, the sensible and latent heats are removed with subsequent condensation. The hot liquid refrigerant then passes to the expansion valve and is ready to cycle again.
While the primary purpose of refrigeration is to remove energy at low temperature, the primary purpose of a heat pump is to add energy at higher temperature. Heat pumps are considered reverse cycle systems because for heating, the operation of the condenser is interchanged with that of the refrigeration evaporator.
Certain chlorofluorocarbons have gained widespread use in refrigeration applications including air conditioning and heat pump applications owing to their unique combination of chemical and physical properties. The majority of refrigerants utilized in vapor compression systems are either single component fluids or azeotropic mixtures.
The majority of refrigerants utilized in vapor compression systems are either single component fluids or azeotropic mixtures. The latter are binary mixtures, but for all refrigeration purposes behave as single component fluids. Nonazeotropic mixtures have been disclosed as refrigerants for example in U.S. Pat. Nos. 4,303,536 and 4,810,403 but have not yet found widespread use in commercial applications.
The condensation and evaporation temperatures of single component fluids are defined clearly. If we ignore the small pressure drops in the refrigerant lines, the condensation or evaporation occurs at a single temperature corresponding to the condenser or evaporation pressure. For mixtures being employed as refrigerants, there is no single phase change temperature but a range of temperatures. This range is governed by the vapor-liquid equilibrium behavior of the mixture. This property of mixtures is responsible for the fact that when nonazeotropic mixtures are used in the refrigeration cycle, the temperature in the condenser or the evaporator has no longer a single uniform value, even if the pressure drop effect is ignored. Instead, the temperature varies across the equipment, regardless of the pressure drop. In the art, this variation in the temperature across an equipment is known as temperature glide.
It has been pointed out in the past that for non-isothermal heat sources and heat sinks, this temperature glide in mixtures can be utilized to provide better efficiencies. However in order to benefit from this effect, the conventional refrigeration cycle has to be redesigned, see for example T. Atwood, "NARBs--The Promise and the Problem", paper 86-WA/HT-61 American Society of Mechancial Engineers. In most existing designs of refrigeration equipment, a temperature glide is a cause of concern. Therefore, nonazeotropic refrigerant mixtures have not found wide use. An environmentally acceptable nonazeotropic mixture with a small temperature glide and with an advantage in refrigeration capacity over other known pure fluids will have a general commercial interest.
Chlorodifluoromethane (HCFC-22) is a currently used refrigerant. Although HCFC-22 is only partially halogenated, it still contains chlorine and hence has a propensity for ozone depletion. What is needed in the refrigerant art is a replacement for HCFC-22 which has similar refrigeration characteristics, is nonflammable, has low temperature guides, and contains no ozone-depleting chlorine atoms.
U.S. Pat. No. 4,810,403 teaches ternary or higher blends of halocarbon refrigerants which are substitutes for dichlorodifluoromethane (CFC-12). The blends have a first component which has a boiling point at atmospheric pressure in the range of -50 degrees C to -30 degrees C, a second component which has a boiling point at atmospheric pressure in the range of -30 degrees C to -5 degrees C, and a third component which has a boiling point at atmospheric pressure in the range of -15 degrees C to 30 degrees C. The preferred blend contains chlorodifluoromethane (HCFC-22), 1,1-difluoroethane (HFC-152a), and 1,2-dichloro-1,1,2,2-tetrafluoroethane (CFC-114). As the reference lists HCFC-22 as a possible refrigerant component, the reference is not teaching refrigerant substitutes for HCFC-22.
As such, the art is seeking new fluorocarbon based mixtures which offer alternatives for HCFC-22 in refrigeration and heat pump applications. Currently, of particular interest, are fluorocarbon based mixtures which are considered to be environmentally acceptable substitutes for the presently used hydrochlorofluorocarbons which are suspected of causing environmental problems in connection with the earth's protective ozone layer. Mathematical models have substantiated that hydrofluorocarbons, such as 1,1,1-trifluoroethane (HFC-143a) or difluoromethane (HFC-32) will not adversely affect atmospheric chemistry, being negligible contributors to stratospheric ozone depletion and global warming.
The substitute materials must also possess those properties unique to the CFC's including chemical stability, low toxicity, non-flammability, and efficiency in-use. The latter characteristic is important, for example, in air conditioning and refrigeration where a loss in refrigerant thermodynamic performance or energy efficiency may have secondary environmental impacts through increased fossil fuel usage arising from an increased demand for electrical energy.
The aforementioned environmentally acceptable refrigerants HFC-32 and HFC-143a are flammable which may limit their general use. These refrigerants are generally regarded as too low boiling fluids to directly replace chlorodifluoromethane (HCFC-22).
In order to overcome the flammability of HFC-32, we blended HFC-32 with 1,1,1,2-tetrafluoroethane (HFC-134a) and the result was zero ozone depletion potential compositions which are useful substitutes for HCFC-22. At high amounts of HFC-32 though, compositions of HFC-32 and HFC-134a are flammable. In order to completely eliminate the flammability of such compositions, we decided to add a third nonflammable component. In adding a third component, we wanted the resulting ternary composition to have a zero ozone depletion potential and have a boiling point comparable to that of HCFC-22. One member from the list of compounds having zero ozone depletion potential and boiling points at atmospheric pressure in the range of -90 degrees C to -60 degrees C is trifluoromethane (HFC-23) which has a low critical temperature; as those skilled in the art know, compounds having low critical temperatures are not used as refrigerants because they do not condense at room temperature and in a refrigerant blend, would be expected to substantially reduce the refrigeration efficiency and capacity of the blend. We were pleasantly surprised to find that in addition to being nonflammable, a blend of HFC-32, HFC-134a, and HFC-23 has refrigeration efficiency and capacity substantially the same as a blend of HFC-32 and HFC-134a.